Polylactate (PLA) is a common biodegradable polymer derived from lactate that is highly applicable to the synthesis of general-purpose or medical polymers. Today, polymerization of lactates produced by microbial fermentation is a method for synthesizing PLA. However, this direct polymerization of lactates can only produce PLA with low molecular weight (1000 to 5000 daltons). PLA with a molecular weight of 100,000 daltons or more may be polymerized from smaller PLA molecules produced by the direct polymerization of lactates using a chain coupling agent. However, this method adds some complications to the process due to the addition of a solvent or chain coupling agent, both of which are difficult to remove. The most widely used method of producing high molecular weight PLA includes the conversion of lactate into lactide as well as the synthesis of PLA using the ring-opening polyaddition of lactide rings.
When PLA is synthesized from lactate, producing a PLA homopolymer is easy. However, it is difficult to synthesize PLA copolymers that have various compositions of monomers and very ineffective in a commercial aspect.
Polyhydroxyalkanoate (PHA) is polyester that acts as an energy or carbon storage molecule in microorganisms when there are excessive levels of carbon but a lack of other nutrients such as phosphorus, nitrogen, magnesium and oxygen. PHA is known as an alternative to conventional synthetic plastic due to its similarity to conventional synthetic polymer derived from petroleum as well as its perfect biodegradability.
To produce PHA from microorganisms, there must be present an enzyme that converts the metabolic product of the microorganism into PHA monomer as well as PHA synthase, which then synthesizes PHA polymer from PHA monomers. To synthesize PLA and PLA copolymer using microorganisms, a system as described above is needed, as well as enzymes capable of providing lactyl-CoA and hydroxyacyl-CoA, which is originally a substrate for PHA synthase.
Further, for economical production of biodegradable polymer, it is crucial to efficiently accumulate PLA and PLA copolymer in the cell. In particular, it is necessary to produce a high concentration of PLA and PLA copolymer through high concentration cultivation. Thus, technology that allows the efficient production of a recombinant microorganism compatible with the conditions described above is needed.